Field of the Invention
The present invention relates generally to the field of fiber optics and in particular to techniques and devices for coupling to the individual cores of a multicore fiber and further, for maintaining low optical loss when coupling to the individual cores of a multicore fiber.
Background Art
Because of the ever-expanding need for large-capacity optical networks, there is increasing interest in the design and fabrication of systems employing multicore fibers (MCFs) as a means for expanding network capacity. The full exploitation of the potential of MCFs has required the development of fibers having a large number of closely-spaced cores, e.g. 7 or more, as well as corresponding fiber-based amplifiers, and other fiber based components, comprising a plurality of cores for amplifying signals carried by respective individual cores in a multicore fiber (MCF) transmission system.
To achieve amplification using a multicore fiber amplifier, it is necessary to separately launch signals from a plurality of input fibers into respective individual amplifier cores. In addition, it is desirable to be able to pump each core separately, in order to maintain a constant or predetermined gain under various conditions of an input signal, independent of input power fluctuations and other parameters.
One technical issue that must be addressed arises from the fundamentally different geometries of typical input fibers and multicore amplifier fibers. For example, it may be desired to provide coupling between 7 standard single-core, single-mode fiber (SSMF) inputs and respective individual cores of a 7-core amplifier multicore fiber (A-MCF), wherein the 7 SSMFs and the A-MCF each have substantially the same outer diameter, e.g. ˜125 μm. A coupler must provide a transition between the cross-sectional area of seven SSMFs and the cross-sectional area of a single SSMF, in a way that allows each SSMF core to be connected to a respective individual A-MCF core.
In addition, a successful coupling solution must also address the issue of mode field diameter. The mode field diameter at each input into the coupler should match the mode field diameter of each respective input SSMF, and the mode field diameter of each core at the coupler output should match the mode field diameter of each respective A-MCF core. A significant mismatch in mode field diameter will result in unacceptably high loss. Thus, a coupling approach in which segments of SSMF are tapered in order to achieve a desired output geometry is unsatisfactory because of the increase in mode field diameter that typically results from tapering.